Bending presses are machine tools of a type that is itself well known. As shown in accompanying FIG. 1, the machine tool comprises a lower table 12 and an upper table 14 that is movable relative to the lower table 12. Usually, the lower table 12 is stationary and the upper table 14 is suitable for being moved towards the lower table 12 under drive from actuators V1 and V2 that act on the ends 14a and 14b of the upper table 14. Usually, the lower table 12 has its free edge 12a fitted with fastener means 16 for fastening bending matrices 18. In the same way, the edge 14c of the upper table 14 is fitted with fastener means 20 for fastening bending punches 22.
A metal sheet or lamination F is placed on the bending matrices 18 of the lower table 12. The sheet F may be of a length that varies widely depending on the circumstances. Under drive from the pistons of the actuators V1 and V2, the punches 22 mounted on the upper table move towards the metal sheet or lamination F placed on the matrices of the lower table. As soon as the punch 22 comes into contact with the sheet, force begins to increase within the sheet as the punch penetrates therein, initially in the elastic range and subsequently in the plastic range, thereby enabling the sheet to be bent permanently.
Because the force is applied to the upper table 14 by the actuators V1 and V2 acting on the ends of the table, the linear load distributed between the two ends of the tables corresponds to the upper table being deformed along a line in the form of a concave arc with deformation maximas close to the midplane of the table. This means that, for bending purposes, at the end of bending, the central portions of the punches 22 have penetrated into the sheet F less than have the end portions. If bending were to be performed on a matrix that, itself, were to remain perfectly straight during bending, then the result would be that a workpiece would be obtained having a bend angle that was wider in its central portion than at its ends. Such a result is naturally unacceptable.
In order to remedy that drawback, various solutions have been proposed for the purpose of controlling these deformations at the edges of the tables by using various means in order to obtain a bend that is substantially identical over the entire length of the bent workpiece.
Conventionally, these solutions involve providing slots, such as the slots 24 and 26 shown in FIG. 1, that are formed in the lower table 12 symmetrically about the midplane P′P of the press. These slots 24, 26 then define between them a central zone 30 of the lower table 12 that is slot-free and that presents a length b, each of the two slots 24 and 26 being of length a. With slots 24 and 26 of conventional type, i.e. that leave between them a slot-free portion 30 of length b, substantially parallel deformations are obtained for the edges of the upper and lower tables 14 and 12.
In addition to the difficulty of proposing a bending press that is suitable for enabling the metal lamination or sheet F for bending to be deformed substantially uniformly over the entire length of said lamination or sheet F, regardless of whether its length is short compared with the length of the tables 12, 14 of the press or, on the contrary, is equal to the length of the tables 12, 14 of the press, there exists an additional difficulty related to managing the deformation of the top edges 24″, 26″ of the slots 24, 26 while the bending force of the moving table 14 is being applied to the stationary table 12, and said force is being taken up on the bottom edges 24′, 26′ of the slots 24, 26, such management of said management being hitherto poorly mastered.